Abstract
The migration and transformation behaviors of heavy metals (HMs), including Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb, during the hydrothermal carbonization (HTC) of sewage sludge (SS) were investigated. The immobilization of HMs during the combustion of solid residual (SR) produced from HTC of SS was also analyzed. With increasing HTC temperature and residence time, the majority of HMs (except As) accumulated in the SR. The residual rate of As in the SR decreased from 73.95% to 56.74% when the residence time was increased from 1h to 3h and reduced significantly from 73.95% to 37.48% when the temperature increased from 220°C to 280°C, implying that numerous arsenic compounds dissolved into liquid phase products. Although the HTC process has a positive influence on the transformation of HMs from weakly bound fractions to the more stable fractions, the exchangeable and reducible fractions of Mn, Zn, As, and Cd in the SR were still high. In addition, the leached amounts of Zn and As were high (14.61 and 6.16 mg/kg, respectively) and showed a high leaching risk to the environment. An increase in HTC temperature and residence time led to an increase of the residual rate of HMs in the combustion residual of SR, implying that the HTC process promotes the stabilization of HMs in the combustion process.
Highlights
Sewage sludge (SS) is a great potential energy resource and has attracted wide attention as a subject of research
After the Hydrothermal carbonization (HTC) process, the moisture content and volatile matter reduced significantly whilst the fixed carbon and ash content increased, implying that the HTC process has a positive effect on the fuel characteristics of SS
The transformation behaviors of Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb in the SS during the HTC process combined with combustion were investigated
Summary
Sewage sludge (SS) is a great potential energy resource and has attracted wide attention as a subject of research. It contains high concentrations of pollutants, including pathogenic bacteria, heavy metals (HMs), and toxic organic compounds [1, 2]. Hydrothermal carbonization (HTC) is a process whereby sludge can be converted into high-density solid fuel under mild temperatures and pressures [8,9,10]. Waste heat from the HTC process can be used to preheat the raw materials and improve the recovery of energy [11, 12]. HTC of SS for high-density and clean solid fuel preparation is a promising technology
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